As a conventional technique of improving the reliability of communication, error correction coding is used. Also, as such another technique, a retransmission technique (e.g. HARQ (Hybrid-Automatic Repeat reQuest)) of retransmitting data that could not be decoded on the receiving side, is used.
In 3GPP, HARQ retransmission control is performed in TB (Transport Block) units. This transport block is formed with a CB (Code Block) group that is an error correction coding unit.
FIG. 1 illustrates a transport block forming method.
As shown in FIG. 1, first, a CRC (Cyclic Redundancy Check) is attached to a transport block data sequence. Then, the data sequence with a CRC is divided into a plurality of partial data sequences corresponding to code blocks, and a CRC is attached to each of the resulting code blocks.
Next, the plurality of code blocks are subjected to error correction coding and rate matching processing (such as puncturing and repetition), and then combined. Thus, the transport block shown in FIG. 2 is formed.
This transport block subjected to that coding processing is transmitted as a codeword, and the receiving side of this codeword performs reception processing supporting the processing explained using FIG. 1. That is, first, decoding processing is applied to the received codeword. Then, whether or not the received codeword contains error is decided (i.e. CRC check) using CRC's attached to the transport block (including the CRC attached to each code block).
As a result of this CRC check, while an ACK is reported from the receiving side to the transmitting side in the case of “OK” (i.e. in the case where there is no error), a NACK is transmitted in the case of “NG” (i.e. in the case where there is error). Here, this NACK indicates whether or not there is error in transport block units, and does not indicate whether or not there is error in code block units.
Then, upon receiving a NACK, the transmitting side retransmits the transport block for this NACK.
As described above, by using a transport block as a retransmission control unit, when a CRC check result related to any code block is “NG,” the receiving side can stop reception processing such as decoding processing for the remaining code blocks. Therefore, the receiving side can eliminate needless processing, so that it is possible to reduce power consumption.
Further, by using a transport block as a retransmission control unit, it is possible to reduce the amount of signaling related to retransmission control, compared to the case of using a code block as a retransmission unit. Signaling related to retransmission control refers to, for example, an ACK/NACK signal, the process number attached to a retransmission unit, and so on.
Also, as a technique used in 3GPP LTE, there is MIMO (Multiple Input Multiple Output). Non-Patent Literature discloses a technique of transmitting a transport block in 4×4 MIMO. FIG. 3 is a block diagram showing a configuration of a MIMO transmitting apparatus adopting the transmission technique disclosed in Non-Patent Literature 1.
In FIG. 3, a transport block (codeword) formed in an encoding section is received as input in an S/P section (serial-to-parallel conversion section). Then, the S/P section separates a plurality of code blocks included in the transport block individually and maps the plurality of partial code blocks, which are obtained from one code block, in respective streams. In FIG. 4, partial code blocks #1A and #2B of code block #1 are mapped in streams #1 and stream #2, respectively.
This MIMO transmitting apparatus adopts the above retransmission control, and therefore can decide a success or failure of transmission only in transport block units. Therefore, upon receiving a NACK, this MIMO transmitting apparatus retransmits all code blocks forming the transport block.
By the way, in MIMO communication, there is a case where communication quality varies between streams depending on the transmission environment. Therefore, if a transport block mapping pattern for a plurality of streams is fixed, a ease is possible where error repeats occurring in a specific part of the transport block.
To solve this problem, Non-Patent Literature 1 suggests a MIMO transmission method of switching mapped signals between streams every retransmission. That is, as shown in FIG. 5, upon the initial transmission, code blocks #1A, #2A, #3A and #4A, which are partial code blocks, are mapped in stream #1, and code blocks #1B, #2B, #3B and #4B, which are partial code blocks, are mapped in stream #2. By contrast with this, in the first retransmission, code blocks #1A, #2A, #3A and #4A, which are mapped in stream #1 in the initial transmission, are mapped in stream #2.
Thus, by switching mapped signals between streams every retransmission, the reliability of symbols in an arbitrary code block is averaged. By this means, it is possible to reduce the possibility that error due to the imbalance of communication quality between streams, repeats occurring in a specific part.